The mushroom cloud of the first test of a hydrogen bomb "Ivy Mike" looms over the Pacific Ocean in 1952. Scientists propose that a new geological epoch, the Anthropocene, began in 1950, marked in rocks and sediments by the fallout from nuclear tests during the 1950s and 1960s. (Reuters)

We're living through one of the most extraordinary events in Earth's history — the start of a new geological epoch, an international group of scientists says.

Welcome to the Anthropocene, everyone.

Geological epochs are long periods of time — typically lasting around two million years — separated by major, global changes to the planet, such as the massive exploding meteor that ended the Late Cretaceous and wiped out the dinosaurs.

Modern humans arose during the Pleistocene epoch, and since the sudden warming that ended the last ice age about 12,000 years ago, we had been living in the Holocene epoch.

But modern human technology has had such a profound effect on our planet that we're now in a new epoch that started during the mid-20th century — the Anthropocene, argues an international group of researchers in a new paper published today in the journal Science.

The boundary between two epochs is visible to geologists as some kind of "marker" between layers of rock, soil or ice that are deposited all over the Earth over time. For example, the Late Cretaceous-ending meteor left a distinct layer of iridium.

In the case of the Anthropocene, scientists note that humans have produced unusual materials like radioactive fallout from nuclear tests in the 1950s and 1960s.

"They've left a permanent record in our sediments and our soils and our glacial ice that's going to be detectable for millennia," said Colin Waters, a geologist with the British Geological Survey and secretary of the Anthropocene Working Group, whose members authored the new report.

"Geologists in millions of years time will look back at and say, 'Something quite incredible happened at this time' and be quite precise about when it happened."

In their paper, the researchers added, "Not only would this represent the first instance of a new epoch having been witnessed firsthand by advanced human societies, it would be one stemming from the consequences of their own doing."

Nobel Prize-winning scientist Paul Crutzen first proposed in 2002 that a new geological epoch called the Anthropocene be assigned to the present to describe the profound changes that humans have made to the planet.

That eventually led the International Commission on Stratigraphy, the scientific body that officially decides when epochs begin and end, to ask a group of geologists, paleontologists and other scientists to look into whether there was enough science to back up that proposal. The Anthropocene Working Group has been working on the question since 2009.

Many markers

University of Alberta researcher Alexander Wolfe
studies the remains of lake microorganisms in
layers of sediments deposited over decades
and centuries, and says he has personally observed
enormous changes marking the past 50 years.
(University of Alberta)

In the new paper summarizing their findings, they list a large number of "markers" that humans have left in rock, soil and ice around the world. In addition to the radioactive fallout, they make a note of:

Pottery

Glass

Bricks

Concrete

Copper alloys

Elemental aluminum (only found as an ore in nature).

Plastics

Black carbon and other particles from fossil fuel combustion.

High levels of nitrogen and phosphorus from fertilizers and pesticides.

The start of new epochs is often accompanied by climate change and mass extinctions, both of which humans are causing now.

"Humans now control several of the fundamental dials or knobs on the planetary system," said Alexander Wolfe, an adjunct professor of paelobiology at the University of Alberta who is a member of the working group and a co-author of the paper.

Wolfe studies the remains of lake microorganisms in sediments deposited over decades and centuries, and says he has personally observed enormous changes marking the past 50 years.

Making new rocks

Nevertheless, he acknowledged that the group has faced some criticism from people who feel the Earth hasn't had enough time to make enough rock to really define a new geological epoch.

"The reality is we've done some calculations and there's the equivalent of one kilogram of concrete produced by humans for every square metre of the planet," he said.

The new epoch isn't official yet. The Anthropocene Working Group still needs to:

Decide exactly when the Anthropocene began.

Decide what formal marker they'll use to define it and then choose a location in which to drive a "golden spike" into the rock at that marker at a place on Earth where the marker is very distinct.

Formally present its arguments to the International Commission of Stratigraphy and have them accepted.

For now, the group suggests making the start 1950 — when humans started having a really major effect on the planet — and the marker of nuclear fallout from Cold War nuclear tests.

"It's an absolutely bomber marker that fits right in the middle of this transition," Wolfe said.